The spectral breadth of conjugated polymers gives these materials a clear advantage over other molecular compounds for organic photovoltaic applications and is a key factor in recent efficiencies topping 10%. But why do excitonic transitions, which a
re inherently narrow, lead to absorption over such a broad range of wavelengths in the first place? Using single-molecule spectroscopy, we address this fundamental question in a model material, poly(3-hexylthiophene). Narrow zero-phonon lines from single chromophores are found to scatter over 200nm, an unprecedented inhomogeneous broadening which maps the ensemble. The giant red-shift between solution and bulk films arises from energy transfer to the lowest-energy chromophores in collapsed polymer chains which adopt a highly-ordered morphology. We propose that the extreme energetic disorder of chromophores is structural in origin. This structural disorder on the single-chromophore level may actually enable the high degree of polymer chain ordering found in bulk films: both structural order and disorder are crucial to materials physics in devices.
The efficiency of an organic light-emitting diode (OLED) depends on the microscopic orientation of transition dipole moments of the molecular emitters. The most effective materials used for light generation have threefold symmetry, which prohibit a p
riori determination of dipole orientation due to the degeneracy of the fundamental transition. Single-molecule spectroscopy reveals that the model triplet emitter tris(2-phenylisoquinoline)iridium(III) (Ir(piq)3) does not behave as a linear dipole, radiating with lower polarization anisotropy than expected. Spontaneous symmetry breaking occurs in the excited state, leading to a random selection of one of the three ligands to form a charge transfer state with the metal. This non-deterministic localization is revealed in switching of the degree of linear polarization of phosphorescence. Polarization scrambling likely raises out-coupling efficiency and should be taken into account when deriving molecular orientation of the guest emitter within the OLED host from ensemble angular emission profiles.
Control of chain length and morphology in combination with single-molecule spectroscopy techniques provide a comprehensive photophysical picture of excited-state losses in the prototypical conjugated polymer poly(3-hexylthiophene) (P3HT). A universal
self-quenching mechanism is revealed, based on singlet-triplet exciton annihilation, which accounts for the dramatic loss in fluorescence quantum yield of a single P3HT chain between its solution (unfolded) and bulk-like (folded) state. Triplet excitons fundamentally limit the fluorescence of organic photovoltaic materials, which impacts on the conversion of singlet excitons to separated charge carriers, decreasing the efficiency of energy harvesting at high excitation densities. Interexcitonic interactions are so effective that a single P3HT chain of >100 kDa weight behaves like a two-level system, exhibiting perfect photon-antibunching.
